Interviewee: Günther Hasinger, Director of Science and Head of the European Space Astronomy Centre
Interviewer: Zan Boag

Zan Boag: As a space scientist, you’ve been gazing deeply into space. You’ve been living and breathing the idea of ‘space’. The universe has been your playground. What is space? How far does it extend? Is it limitless?

Günther Hasinger: It is a little bit like the overview effect that the astronauts get when they leave the Earth and then they see the Earth as a small blue dot in a big surrounding universe. But as an astrophysicist, a space scientist, I actually have a perspective which is even larger. So, basically, our whole solar system is just one out of many. Our whole galaxy is one out of many, and so in principle you could imagine that space, as such, is infinite. It is very likely not infinite, but it is bigger than anything we can imagine. And the point is that there is a horizon. When you ask what is at the end of the universe or the end of space, it turns out that the end of space as we see it is only an optical illusion. Space goes much further, beyond where we would ever be able to see.

And you can imagine this, like when you stand on the coast and you look at the horizon, you think that the Earth is finished there, right at the horizon, but you know very well that when you see ships coming up, they come from beyond the horizon. And when you go to the horizon you see another horizon, and another one. The same is true for our universe. So, what we see as the boundary of space right now with the Microwave Background glow, is only a horizon. And if we would be able to fly there, we would see billions of more times the same universe.

In principle, if you take our whole Earth and you put a needle down, the tip of the needle is roughly what we see from our current universe and there is so much more universe out there which we cannot see. The reason we cannot see it is because the lifetime of the universe is finite, and therefore light did not have enough time to travel to us from those distant regions. It’s not that there is an end to this, it is basically just that we don’t have any information because light doesn’t reach us from there.

For a better understanding, one first would have to guide people through the huge dimensions. For instance, the time the light takes to travel to us. The light from the Sun takes eight minutes to travel to us. From the outer solar system – Jupiter, Saturn – it takes hours to get to us. From the next star, it takes four years to come to us. And then from the centre of the galaxy, it takes 30,000 years to reach us. The next galaxy is about 170,000 light-years away and the next big galaxy a million light-years.

And so, you work out into space by the time the light needs to travel to us, and at some distance you reach the point where the light that travels to us needs longer to get to us than the age of the universe, and therefore the light has never reached us.


There’s an article on the ESA website with the headline, ‘Galactic crash may have triggered solar system formation’, in which it states that the formation of the Sun, the solar system, and the subsequent emergence of life on Earth may have been a consequence of a collision between our galaxy, the Milky Way, and a smaller galaxy called Sagittarius. Could you elaborate a little bit on this potential beginning of our solar system?

Indeed, the satellite Gaia, which is our real work horse out there, is currently the most prolific machine. About three papers per day are coming out, so it’s really a flurry or a feeding frenzy that the astronomers are currently having. Gaia really gives us spectacular insights into the history of the Galaxy. And, in particular, it also is able to deduce how the Milky Way has been built up from, in principle, from crashes with other galaxies. So, we have identified one major crash, which was about 10 billion years ago, which is almost twice as old as our own Sun. That was before this event that you mentioned happened. That was a rather big galaxy that crashed into the Milky Way and it shook up the whole disc of the Milky Way and made the stars dance much faster.

And then there is this fascinating dwarf galaxy, Sagittarius Dwarf it’s called. Sagittarius Dwarf, because currently it is visible in the direction of the galaxy centre in the Sagittarius constellation. But through the Gaia data astronomers could dig into the history of that Sagittarius Dwarf, and it is orbiting the Milky Way in a large arc, in a large orb. Every once in a while it dives through the Milky Way, and when that happens it kind of shakes up the environment and is triggers new star formation.

What Gaia has done, was to measure the amount of star formation as a function of time, and there are distinct peaks in that star formation history. So, there are some times where there’s a huge amount of star formation, and then other times where there is little. It turns out that these peaks of star formation correspond more or less directly to the orbit of the Sagittarius Dwarf. Whenever the Sagittarius Dwarf crashes through the Milky Way, it triggers new waves of star formation. Currently it’s doing that as well, but about 5.7 billion years ago, that was when we could first identify that. That was probably the first interaction of that galaxy with our Milky Way. That is exactly the time when the solar system was born.

We don’t have a one-to-one proof that this is the case, so we cannot really say that Sagittarius triggered that event, but it is quite a good coincidence. That’s very likely, because we are seeing that these galaxy crashes are triggering star formations. It’s very likely that it has also triggered the formation of our own solar system.


In an interview, you said that what we’re learning is basically the history of the Milky Way. You’re drilling back billions of years into the history of time, much like space archaeologists. You’ve learnt a lot about the history of the galaxy and you can speculate about what may have happened, but can you predict the future of the Milky Way?

Yes, that’s also a very interesting topic. It’s all basically connected to crashes with other galaxies. Whenever galaxies collide they change their shape, and they evolve that way. They grow or they change appearance. There’s a very interesting event coming up, but only in about five billion years, so another lifetime of the Sun and the Earth is necessary before that will happen. That is when the Milky Way and the Andromeda Nebula will crash into each other. The Andromeda Nebula is a very fascinating galaxy in its own and you can see it by the naked eye or with binoculars in the Northern Hemisphere, in the constellation Andromeda.

It turns out that the Milky Way and Andromeda are coming closer to each other. While most of the other galaxies are moving away from each other, the Milky Way and Andromeda are approaching each other. Gaia has been able to accurately map the motion of these galaxies. There’s a third galaxy in the dance, called M33. Gaia has traced out the motion of all of these three galaxies and can now quite accurately predict that the third galaxy will not crash into either of the other two. That leaves the way free for the Andromeda Nebula to really merge with the Milky Way, and they can predict quite accurately when that will happen.

You can imagine that when you are an astronomer in five billion years, there’s this galaxy coming towards us and the sky will all of a sudden be filled with a huge beautiful spiral galaxy and then after a while, when the two galaxies merge, all the stars shake up. It’s not a danger for the planets, they will survive, but the whole shape of the Milky Way will change. The Milky Way will become an elliptical galaxy, a big round ball of stars. That will completely change our environment. The sky will look completely different then.

You take it for granted that the universe is constantly evolving and changing, but for the everyday person, who’s sitting down here and they have their everyday worries and ideas, they look up to the stars above and they see something that appears to be quite constant. The stars are up there in their positions that we imagine them to be. We have the cycle of the seasons and the years going by. But this idea that the universe is ever-changing is something that is probably a little bit foreign to most of us.
I think for a long time, and even Einstein a hundred years ago originally was convinced that the universe was static and all of his ideas that indicated that there should be motions and should be changes, he put them into the dumpster first.

And then only later, when the astronomers discovered that the universe is changing and the galaxies are moving away from each other, then he said it was the greatest blunder of his life that he basically didn’t realise that his own laws would predict that the universe is changing.

I think it’s connected to the fact that we, as humans, are always trying to judge everything according to our scale and experience. So, we judge everything that is connected to time in years, which we can kind of overlook. That’s the reason why the change of the years, like we had just recently, is so important.
Sizes are typically judged in metres because that’s what we can judge. This is true both for the huge universe, but also for the minute universe. As soon as you get to very small things, you need a microscope, you get all of a sudden a completely new fascinating world that is not part of our own. The same is true when you go out and to go to larger and larger distances, you realise that there is something out there which is completely inhuman, and this is space.

And time scales, it’s the same. I mean, you have to basically set yourself into a completely different time frame, a little bit like the people who are digging into the ground to study the history of our Earth. You are then all of a sudden getting to time scales of billions of years. And then you see that the mountains are changing. It’s not just the sky, when you look at the Earth itself you see that the Alps are still growing, but they are growing on time scales which are completely unrealisable for humans.


Well, that’s the arrogance of the anthropocentric view.



You wrote an interesting book, Astronomy’s Limitless Journey: A Guide to Understanding the Universe. In it you cover all sorts of topics that help us make sense of what the universe is, and what it’s made up of. One point of interest for me is dark matter and black holes, which conjures up all sorts of fantastical ideas. In your book you touch on these regions of space. What are they and how do they differ from one another? What effect do dark matter and black holes have on the universe?

This is a very topical question, because I’m currently working on trying to understand whether black holes and dark matter is actually the same thing. I mean, in my book there’s no inkling of that, but it is getting more and more fascinating, this idea, that dark matter may actually be made out of primordial black holes, which would be closing the circle big time.

Let me start with black holes first. Black holes have been postulated by Karl Schwarzschild. Einstein never believed in them, but Karl Schwarzschild, who solved the Einstein equation, came to the conclusion that there must be a type of object that is so compact and so dense that it doesn’t let light out, it doesn’t let the light escape. And, more than a hundred years before, John Michel and Pierre Simon Laplace already speculated about dark stars, which are so heavy that they even eat their own light. You can think about that, when you look at the Sun, you know that when you see a star behind the Sun it is slightly gravitationally bent inward. That was the prediction that Einstein had, that the stars behind the Sun should be slightly pulled inwards by the gravitational field of the Sun, so that when a light ray is passing by the surface of the Sun it is slightly bent.

This is this so-called gravitational lensing effect. With the Sun it is only a minute amount that the light is bent. But then if you do a thought experiment, if you reduce the diameter of the Sun by a factor of two, so you just shrink it to half of its size, but you keep the mass, then the light is bent twice as much as for the Sun. And indeed, there are stars that are smaller than the Sun, and heavier, and so the light would be bent totally. You can extend this thought experiment until you reach a diameter where the light is basically bent 360 degrees. So, every light ray that is passing that star is bent around the star and doesn’t leave the star anymore.

You come to the conclusion, and it was Schwarzschild who discovered that, if you squeeze the Sun down to a radius of three kilometres, about the size of a small city, then the Sun would become a black hole. A black hole is, in that sense, nothing special, nothing dramatic, it is just a very compact object which is so heavy that it lets the light circle around it and not escape anymore. If you would do the same thing to the Earth, I mean, it’s a little bit crazy, but if you would squeeze the Earth down to a one-centimetre size then the Earth would become a black hole.

In principle, there could be planetary small black holes, but they are only tennis ball or ping pong ball size. If you squeeze the whole galactic centre with billions of stars into a radius which is roughly the radius of the orbit of Mercury, then that galaxy would become a black hole.

And so, for quite some time, for almost a century, black holes were only a kind of speculation of theorists, a figment of imagination. But then, in the sixties, there were the first discoveries of very fascinating objects which radiate very much in X-rays or in radio, and they are hugely variable. And people realised that these are very likely black holes which are accreting matter from other objects, so that you can see them. When matter is falling onto such a black hole, it is heated up and radiates its last cry of help in the form of electromagnetic radiation, in the form of X-rays, infrared, radio.

This way we have discovered the existence of black holes. Just last year the Nobel Prize was given to Reinhard Genzel and Andrea Ghez for measuring the mass of the black hole in the galactic centre, where you can see the stars are swirling around the object, but there is nothing there, nothing visible. They could determine that there’s a four-million-solar-mass black hole.

I was fascinated by black holes throughout my whole career because they are very loud X-ray emitters, so they are very visible in X-rays. When you have an X-ray telescope and you point it to the sky, you see zillions of black holes everywhere.

That was basically one side of the story, the black holes. Now, let’s quickly talk about dark matter. In the beginning, dark matter is something completely different. It portrays itself by the fact that the stars in our galaxy, and also the galaxies themselves in the clusters of galaxies, are moving much faster than they are allowed. If you take all the stars of our galaxy together, you can measure the mass that is in the stars, and then when you see how fast the stars are moving, the mass of the stars is not sufficient to keep them from flying away.

There is the centrifugal force and the centripetal force, that means if you sling something around in a circle then it tries to leave. The stars in our galaxy are moving about five times faster than they should. From the very beginning, in the thirties already, people have postulated that there must be some hidden, unseen matter which is keeping the galaxies together and which is keeping the galaxies in the clusters of galaxies. That unseen matter has been called dark matter because it is there, it’s clearly showing up by its gravity, but it’s not showing up in light.

Then for a long time, for decades, people were looking at what this dark matter could be. You also see fascinating phenomena like gravitational lensing, where you see galaxies behind clusters of galaxies, which are amplified through lenses until you can measure quite accurately the amount of dark matter which is there, the distribution and so on, but nobody has so far found an inkling of what its nature is, I mean, what particle is responsible for the dark matter. That is going now hand in hand with black holes, because with black holes there are a few phenomena which we don’t understand.

We find black holes in places and in times when they should not be there. Very early in the universe there must already have been quite heavy black holes, and we find different types of black holes which should not be there. One of the most recent fascinating speculations is that we may even have a black hole in our own solar system. One putative additional planet, which is called the Planet X, may possibly be a black hole. Finally, the gravitational wave mergers of rather massive black holes are another piece of the puzzle.

These two things, black holes and dark matter now tie together because there is a new theory which postulates that very early in the universe, before anything else was formed, the universe already created black holes of different sizes. These black holes are now regarded as one potential source of the dark matter. So, the dark matter and the black holes could be the same thing.


Are they, in a way, the glue of the universe?

They are holding the galaxies together and they are holding the galaxies in the large-scale structure together. But they don’t hold the universe together as a whole because there’s another force, which we discovered only a decade ago, and that is dark energy. Dark energy is actually fighting against dark matter. Dark energy is pushing out and dark matter is pulling in. It turns out that in the end dark energy is stronger than dark matter, and the universe, as a whole, is expanding even farther. So, the galaxies are flying away faster tomorrow than they do today. The galaxies themselves are not flying apart. The stars in the galaxies and the galaxies in the clusters are not flying apart, but the large-scale structure of the universe is growing, and the universe itself is basically expanding.


And this is due to dark energy?

Very likely, yeah. So, I mean, we were always talking about the big bang as if it is an explosion that was there in the beginning and then nothing else happened, but now we know that the force which is pushing out the galaxies is still acting today. In a sense the big bang is still active today, and the force that is pushing the galaxies apart is still accelerating them. It looks like that is the reason for the dark energy, or the dark energy is the reason for the expansion of the universe.


Now, to the future of space science: where are we heading? What are the main areas of focus over the coming decades? Where do you think the energy of ESA will be concentrated over the next 10 or 20 years?

At some point I jokingly said there’s only one question, and that is, “Where did we come from and where will we go to?”

We have actually discussed that question, but then you can break that question down, because it applies to different scales. There are, I think, three very important lines of research. One is cosmology. That means, where do we come from? Where do we go to? The whole universe, the galaxies. The other line is on the scale of the solar system: how did the solar system form and what will be its future? That is basically solar system exploration and the study of exoplanets. We are finding thousands of planets around other stars, and so I think exoplanets will dominate the study for the next decade.

And then a very important, even smaller scale, life itself, a very, very exciting question is, how did life form and what will be the future of life? So, in each of these three threads, it’s basically always looking in the past and trying to understand what the future is. I believe that these three strands, cosmology, black holes, dark matter, dark energy, is one part, exoplanets is the other part, and the search for life is the third big question.


They aren’t trivial questions. Private individuals and organisations have joined in, hunting for answers to these questions. A lot of it is based around exploration or potentially the colonisation of other planets. What does this mean for established organisations like ESA and NASA, when you have these smaller organisations and private individuals competing with you?

First, let’s break it down. In space science it is really great to have diversity, we have friendly and fruitful competition. That means, the more people that are thinking about big questions, the more answers we will get. And so, for instance, NASA and ESA are already collaborating in an excellent way. This year we will launch the James Webb Space Telescope, which is the biggest enterprise ever in astronomy. And, together, so it’s another strategic mission, but either we’ll launch or we’ll partner with it. And more and more smaller countries are coming in as well and they also do space science.

China is not a small country; they are a growing power. Russia has always been there. But also Japan, and there are more countries like India, South Africa, the United Arab Emirates and so on, who are getting into the scene. I think partly also because of strategic thinking, similar to the space race in the sixties, space is regarded as very important, not just a commodity, but a strategic asset. Now, when it comes to private companies, space is not just space science. Space is also an infrastructure which is very important. Our daily lives are completely affected by space, whether it’s the weather forecast, the understanding of climate change, navigation, communication.

A lot of the internet trade goes through space and quantum technologies are getting very important. And so, there is clearly also a very strong interest in earning money from space. And so, when you say private companies are coming in, they are not just coming in for fun, they really want to make big business out of space. I think this is a good thing, because then as soon as people see that they can earn money, there is competition, things get better, cheaper, and so in the long run I believe this is a good thing. There’s one big dream that I think is driving people like Jeff Bezos or Elon Musk, and that is to do space tourism, to bring people to the Moon, to Mars, and so on.

From my point of view, I would say this is still premature. I don’t think that you will be able to make big money out of space tourism yet. But, on the other hand, a hundred years ago the same was probably true for airplanes. At some point there will be space tourism. What we are a bit concerned about is that this way, also with the big constellations and so on, space is getting very, very crowded. It’s becoming a danger, because people are leaving space junk out there, and the more space junk you have, the more dangerous it is to fly new spacecraft.

And, also, astronomers are a little bit worried that the sky is significantly affected by these large constellations of spacecraft. But I think this can be managed. It requires a new space law, international agreements about how to deal with that. I would say, competition is fruitful and it’s a good thing.
We also sometimes call it coopetition, because it’s somewhere between cooperation and competition. When you think about the big private companies, they are currently still very much working with NASA and even ESA is discussing some cooperation with various private companies.

You spoke earlier about space tourism, which seems to be a focus for some of them, but there is also talk of colonising space. So not just for tourism, but to actually live, be it on Mars or on the Moon, to have permanent bases there – not simply for tourists to go up. There are a lot of problems with being in space: psychological issues and physiological issues. It’s not so easy to sustain one’s mind and body in another realm, but perhaps the biggest problem is that wherever we go, we take ourselves with us. How likely are we to recreate the problems that we face here on Earth?

Let’s break that down first. I believe it is still quite dangerous out there. When the Apollo flights happened, everybody was excited and happy that astronauts were going to the Moon. Nobody realised that the radiation from the Sun, in particular, and also from outer space, really posed a significant danger. For some reason, the Apollo astronauts were extremely lucky that there was not a single magnetic storm or solar eruption during their missions, because if you are unlucky and you are hit by one of these storms from the Sun, you may well get very ill or maybe even die. The whole problem of radiation safety for space flight has become a much bigger issue than people realise. So now, when you travel long distances, like to Mars, or if you want to stay on the Moon for a long time, you have to have shelters, both in the spacecraft and also on the surface of the Moon or Mars. People think about, for instance, going into caves or living underground, because the radiation can be quite harsh. When you are thinking about rockets or spacecraft, then you have to provide some shielded space where people could shelter in case of a storm or a sub storm. And so it is actually much less pleasurable than you think.

That’s also the reason why I believe space tourism has a long way to go. If you really are creating outposts on any solar system body, we have to carry a lot of armour with us, or have to dig deep into the ground in order to be safe. I have the feeling this is not really the paradise that people are looking for.

It will be similar to putting an outpost on the South Pole, for instance, which we could already do now, and people are doing. And people are getting used to staying together in very confined spaces for half a year or a year, but it is not, I think, a nice thing to do for day-to-day tourists or people who are looking for different pleasures. I’m afraid that until we actually are able to go completely to another friendly planet, which would be possible in maybe 500 or a thousand years, but until then I’m afraid that space travel will still mainly be limited to a few people and to exploration or very dedicated specific things.

This is a little bit like training for a pilot for fighter planes. You need very special skills and also very special psychology. You need special training to do that. And then people are also trained for this particular question about how to deal with other people in very confined spaces and so on. It’s not fun.


No, I can’t imagine it’s easy. Well, given it’s probably quite difficult for us, under the current conditions, to flourish in outer space, perhaps we’re going to have to start dealing with some of the problems that we face on Earth.

Yes, indeed. And some people say, “Why do you go out to space when you have so many problems here on Earth?” But that’s not the real perspective.

Because learning from space is hugely important for the Earth. For instance, the whole science of climate change originally came from looking at the stars, and from Venus and things like that. A large part of the technological development, which was necessary to sustain our society, also came from space. Like the meteorological or the communication or the navigation aspect, they all originally came out of basic research into fun things or things that are not as important. I think both have to coexist, but we have to basically turn all of our knowledge and all of our attention to the fact that we should not destroy our Earth. That’s much more valuable than to look for a place to live elsewhere.


From which mode of exploration do you think we’re going to learn the most about ourselves and our place in the universe?

I believe that robotic exploration in the solar system will be very powerful to learn whether there is life or pre-forms of life out there, which will then tell us how life on Earth has been formed, and will also tell us a little bit about what we could do to preserve life on Earth, for instance. And then also I believe that distance viewing, telescopic exploration will still be very powerful. On the other hand, the crewed exploration, as we call it, with humans, is also I think very important. Not just for preparing ourselves to live in space. I mean, it’s clear at some point you would like to fly to other planets and explore them personally, but also there is a lot of stuff that you can do in space that you cannot do on ground.

You can even do medical research. You can develop new medicine. Currently, for instance, there is COVID-related medicine flying on the International Space Station. I believe that crewed research will still be important in the future. Crewed exploration, to go wherever we are able to go is really a great challenge, and is also a lot of fun. But I believe that robotic exploration and also astrophysics and cosmology will really be very important for decades.


For you, personally, over the decades you’ve been gazing out into the universe, gazing at the stars, it would have formed who you are, and it would have changed you over the course of the years. What have you learnt about yourself through years of gazing at the universe?

I have learned about myself and my environment, that when you look at things in a different dimension the day-to-day problems become less important. So, for instance, whether you have a red or a black number on your bank account is very important for you personally, but the universe doesn’t care about it. And so, for some of the decisions and some of the things that we do, we should actually take this bird’s eye view and look at the big picture. Then, when you look at the big picture, then indeed a lot of the nitty gritty, even the wars on Earth, become less important if you look at the Earth as a whole, or if you look at the universe as a whole. I believe that, for instance, the cooperation of nations in space, who are in principle enemies and fighting against each other, but in space we can still work with each other; it’s hugely important, also politically.

So, for instance, in the Cold War, we have worked together with the Russians, between Russia and Germany, where I was, or also between Russia and the US. I believe that these human interactions between people who are living in different systems in the end are hugely important for a peaceful coexistence, and they were probably part of the reason why the Iron Curtain fell without any thunder.

And so, I believe getting into this remote perspective and looking at us from a distance is something that is relatively easy for astronauts and cosmologists, and I think day-to-day people could learn from that as well.

From the Space edition, which can be purchased in hard copy or digital format here.

Günther Hasinger is Director of Science and Head of the European Space Astronomy Centre (ESAC). Hasinger was awarded the Leibniz Prize, the most significant research prize in Germany, and the international Committee on Space Research (COSPAR) Award for his outstanding contributions to space science. He is the author of Astronomy’s Limitless Journey, the winner of the Wilhelm Foerster Prize for public dissemination of science in 2011.